Projection lens unit and rear projection type image display system

Abstract

An inexpensive projection lens unit in a rear projection type image display system, having a short focal distance (wide angle of viewing field), high focus and high contrast, and as well, including a power lens made of a glass material having a high refractive index in a range from 1.63 to 1.70 and an Abbe's number of not less than 50 with an optimized incident and emergent configuration in order to satisfactorily correct aberration with a enhanced focusing performance even with a shortened focal distance. Further, the power lens having a high refractive index can exhibit a high power, and accordingly, can carry out satisfactory aberration correction by optimizing the aspheric degree of a plastic lens even though the length of back focus is increased for enhancing the contrast.

1. A projection lens unit having a plurality of lens elements for magnifying and projecting an original image displayed on an image source, onto a screen, comprising:
 a first lens group including a meniscus lens which has a positive refractive power and which has a center part configured so as to be convex on the screen side thereof; 
 a second lens group including a power lens having a highest positive refractive power among the plurality of lens elements; 
 a third lens group composed of a single lens element having opposite side surfaces which are convex near an optical axis thereof, having a positive refractive power, and having a peripheral part which has a meniscus convex configuration on the image source side thereof, and 
 a fourth lens group having a lens surface which is concave on the screen side thereof and having a negative refractive power, 
 the first, second, third and fourth lens groups being arranged in the mentioned order from the screen to the image source, 
 wherein the power lens included in the second lens group has a refractive index in a range from about 1.63 to 1.70. 
 
   
   
     2. A projection lens unit as set forth in  claim 1 , wherein the power lens has a refractive index in a range of about 1.635 to 1.660. 
   
   
     3. A projection lens unit as set forth in  claim 1 , wherein the power lens has a refractive index in a range of about 1.638 to 1.639. 
   
   
     4. A rear projection type image display system comprising:
 an image source; 
 a screen; and 
 a projection lens unit comprising a plurality of lens elements for magnifying and projecting an original image displayed on the image source onto the screen, 
 the projection lens unit comprising: 
 a first lens group including a meniscus lens having a positive refractive power and a center part which is configured so as to be convex on the screen side thereof; 
 a second lens group including a power lens having a highest positive refractive power among the plurality of lens elements; 
 a third lens group composed of a single lens element having opposite side surfaces which are convex near an optical axis thereof, having a positive refractive power, and having a peripheral part which has a meniscus convex configuration on the image source side thereof; and 
 a fourth lens group having a lens surface which is concave on the screen side thereof, and having a negative refractive power, 
 wherein the power lens included in the second lens group has a refractive index in a range of about 1.63 to 1.70. 
 
   
   
     5. A rear projection type image display system as set forth in clam  4 , wherein the image source is a cathode ray tube.